1. National Institute for Medical Research, Mill Hill, London NW7 1AA, UK
2. Institute of Infectious Disease and Molecular Medicine, University of Cape Town, Rondebosch, Cape Town 7701, South Africa
3. Department of Infectious Diseases, Addenbrooke’s Hospital, Cambridge CB2 0QQ, UK
4. Department of Medicine, Addenbrooke's Hospital, University of Cambridge, Box 157, Cambridge CB2 0QQ, UK
5. Department of Environmental and Occupational Health, Center for Global Public Health, School of Public Health, UMDNJ, NJ, USA